Known electrical storage devices having high energy densities, such as electric double layer capacitors and lithium-ion secondary batteries, are manufactured by the following method.
First, active materials, such as activated carbon, a lithium composite oxide, and carbon, are applied to collector foils formed of metallic foils, such as aluminum foil and copper foil, to form electrode sheets.
Two electrode sheets are arranged such that their active material layers face each other. A separator sheet is placed between the two electrodes in order to prevent a short circuit due to direct contact of the two electrodes. The electrodes and the separator are wound or stacked and form a layered body that includes a plurality of pairs of a positive electrode and a negative electrode.
An aluminum tab or a nickel tab is welded to the positive electrodes and the negative electrodes as an outer electrode, for example, by ultrasonic welding. Thus, electrode groups are obtained.
These electrode groups are placed in an exterior, such as an aluminum can or an aluminum laminate film. After an electrolyte solution is supplied to the exterior, the exterior is sealed to complete a known electrical storage device.
However, it is difficult to further reduce the size and height of electrical storage devices by such known methods.
Patent Document 1 discloses an electric double layer capacitor described in detail below as an electrical storage device that can be further reduced in size and height.
An activated carbon electrode layer is formed on aluminum collector electrodes. The collector electrodes are arranged such that their activated carbon electrode layers face each other. A heat bonding portion that may be formed of modified polypropylene or modified polyethylene and can be melted by heating is provided in advance on the peripheries of the collector electrodes. After a separator is placed between the collector electrodes, an electrolyte solution is supplied between the collector electrodes. The heat bonding portion is heated to seal the collecting electrodes (thermocompression bonding), thereby forming a layered body. A unit cell is formed in such a manner, and an electric double layer capacitor is completed.
The heat bonding portion has a function of maintaining the shape of the layered body and preventing the short circuit between the collector electrodes. Thus, the electrical storage device can be further reduced in size and height.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-313679
In the electric double layer capacitor (electrical storage device) described in Patent Document 1, the peripheries of the collector electrodes are completely sealed. Thus, after the layered body is formed by melting the heat bonding portion, the electrolyte solution cannot be supplied to the layered body.
The electrolyte solution must therefore be supplied between the collector electrodes before heating the heat bonding portion or before forming the layered body. This may cause a problem that the heat during the thermocompression bonding changes the quality of or volatilizes the electrolyte solution.
There are also difficulties in handling because the electrolyte solution may leak and adhere to apparatuses while the electrodes are stacked (the collector electrodes) after the supply of the electrolyte solution and before thermocompression bonding.
Furthermore, since the electrolyte solution is sealed between the collector electrodes of the electrical storage device, a gas generated by the decomposition of water or impurities in the electrolyte solution is not discharged from the layered body. This may cause the expansion of the layered body (between the electrodes), a high impedance (internal resistance) or a low capacity, or separation between the components, for example, between the collector electrodes and the active material layers.